1. Field of the Invention
The present invention relates to an infrared analyzer and, more particularly, to an infrared analyzer having an improved calibration capability.
2. Description of Related Art
Infrared analyzers, used in the prior art, direct infrared rays from a light source through a sample cell into which a sample gas is introduced. The difference between a measured signal that is proportional to the energy of an infrared ray having a characteristic of absorption in a wavelength range of a specific ingredient gas and a comparative signal that is proportional to the energy of an infrared ray having a wavelength that is not absorbed, or at least subject to a negligible absorption, is determined. This difference can be amplified to determine the concentration of the specified ingredient gas from the sample. To ensure the accuracy of this measurement, both a zero calibration and a span calibration must be periodically conducted on the instrumentation at frequent intervals. In the process of providing a zero calibration, the sample cell is evacuated or a "zero" gas state is introduced into the sample cell, and after an indication of a stabilization of the system, measurements are taken to reflect the characteristics of the instrument system per se. Subsequently, a span gas of a known purity is introduced into the sample cell, and again after a stabilization period, a span calibration is then conducted. These measurements can then be utilized to ensure an accurate calibration of the infrared analyzer.
A problem has existed in that an expensive span gas having a high degree of purity must be used at every span regulation in order to verify the gas calibration. Thus, the cost of the calibration can become relatively expensive and time consuming.
Alternative calibration methods, such as a mechanical calibration method, have been suggested in the prior art. In this method, the intensity of the light passing through the cell is physically reduced by means of a metallic plate and a filter to thereby change the quantity of the light incident upon the sensor detector. The characteristics of the metallic plate and the filter are known, and thereby a comparison with the output of the infrared analyzer can determine whether the system is appropriately calibrated. In the mechanical calibration method, numerous problems can occur, such as a misalignment of the metallic plate, and these problems can directly affect the sensitivity of the measurement system in a subtle manner that may not be easily discovered and may be treated as an instrument error. Additionally, in the case where a filter is used to reduce the intensity of the light passing through the sample cell, the filter itself can become physically stained or damaged, and can thereby change the quantity of the light. As can be appreciated, the mechanical calibration method, while less expensive than the use of an expensive span gas, still exhibits numerous disadvantages that plague the prior art.
A Japanese Patent Application Laid-Open No. Showa 61-20840 sought to solve the disadvantages of the above-described gas calibration method and the mechanical calibration method. This patent disclosure teaches a method wherein a zero gas state is measured, an input is then changed to the same extent as the absorption by the gas, and an amplifying factor is changed so that the change in the input signal may amount to an appointed change to conduct the span calibration. This method, however, still requires the span regulation to be conducted after the zero-point regulation, and increases the time period of the calibration.
Accordingly, the prior art is still seeking to optimize the calibration of the accuracy of infrared analyzers in an economical and time efficient manner.